Physically-based volumetric rendering is a model in computer graphics that mimics the real-world interaction of light with 3D objects, where the 3D objects are described by a volumetric dataset. Physically-based volumetric rendering based on Monte Carlo path tracing is a rendering technique for light transport computations, where the natural light phenomena are modelled using a stochastic process. Physically-based volumetric rendering models the inter-reflection of light between positions within a volume and can produce a number of global illumination effects by providing a solution to the rendering integral equation. Physically-based rendering hence results in more realistic images, as compared to images produced from traditional volume rendering methods, such as ray casting, which do not produce global illumination effects. Such effects include, for example, ambient light occlusion, soft shadows and colour bleeding. The increased realism of the images can improve user performance on perceptually-based tasks. For example, photorealistic rendering of medical data may be easier for a radiologist, a surgeon or a therapist to understand and interpret, and may support communication with the patient and educational efforts.
Medical images produced by physically-based volumetric rendering may be used for example, for diagnosis, teaching, patient communication etc. However, evaluation of the rendering integral equation in physically-based volumetric rendering based on Monte Carlo path tracing may require many, e.g. thousands, of stochastic samples per pixel to produce an acceptably noise-free image. Depending on the rendering parameters and the processor used, therefore, producing an image may take on the order of seconds for interactive workflows and minutes or even hours for production-quality images. Devices with less processing power, such as mobile devices, may take even longer. These rendering times may result in overly long interaction times as a user attempts to refine the rendering to achieve the desired results. In particular, such long rendering times make current physically-based volumetric rendering completely unsuitable for interactive applications where high image quality as well as high interaction performance have to be achieved. For example, visualisation of a volume with a virtual reality (VR) device is not plausible with an acceptable quality or refresh rate of the resulting images.
Some attempts have been made to decrease the time taken for an acceptable image to be produced by physically-based volumetric rendering. One approach is to simplify the underlying rendering, such that the complex physically-based rendering algorithm is replaced by an algorithm that approximates it. For example, features such as shadows can be added to classic volume rendering.